1. Field of the Invention
The present invention relates to digital loop carrier (DLC) systems and, more particularly, to a line unit interface circuit for interfacing line units with common equipment in a digital loop carrier line shelf.
2. Description of the Prior Art
The Synchronous Optical Network (SONET) standard (American National Standards Institute Standard T1.105-1988 entitled "Digital Hierarchy Optical Interface Rates and Formats Specification") which is being adopted within the United States and elsewhere defines the standard for the transfer of information by means of optical fiber According to the SONET standard, an optical carrier level (such as OC1, OC3, OC12 and OC48) signal is a signal that results from an optical conversion of a synchronous transport signal (STS) operating at the same transfer rate. An STS1 level signal is defined as the basic building block signal, with a high-speed transfer rate of 51.840 Mb/s, and is equated to an OCI level optical signal. With high-speed transfer rates there is a need for multiplexing and demultiplexing information associated with lower-speed telephony standards to and from the high-speed transmission lines. Examples of such lower-speed standards include the digital signal standard, or DSX standard (where `X` is an integer, such as 0, 1, 2 and 3). The DSX standard is commonly used in telephony with DS0 directed to subscriber level signals that operate at 64 Kb/s, DS1 directed to lines operating a 1.544 Mb/s, DS2 operating at 6.312 Mb/s, and DS3 operating at 44.736 Mb/s.
In order to access the high-speed transmission lines, network elements are required for transferring and grooming, i.e., segregating, subscriber information channels between the lower-speed transmission lines and the higher-speed transmission lines. These network elements may take on several different forms for providinq transfer of information between various standard transmission rates. In order to take advantage of the wider bandwidth available on the high-speed lines for various applications such as data transfer, a means was required to combine DS0 channels to provide wider band facilities.
A family of access products has been developed by the Assignee of the present invention. These access products allow slower transmission lines to access the higher-speed optical transmission systems. These access products use an internal multi-link serial bus (SBI) operating at a rate of 4.096 Mb/s to transport information, signalling and processor commands U. S. patent application Ser. No. 351,458 filed May 12, 1989 entitled "Serial Transport Frame Format Method", which is commonly assigned with this application, describes this unique serial bus and is incorporated herein by reference. Two access products are also described in the aforementioned patent application, said access products being a Terminal Multiplexer, adapted to interface a high-speed carrier with DS1 level transmission lines for reception and transmission of high-speed signals in one direction only. An Add/Drop Multiplexer (ADM) is also described in the aforementioned patent application and is designed to interface a high-speed carrier to DSI level transmission lines for reception and transmission in each of two directions.
FIG. 1 illustrates the use of access products in a telephone transmission system. Two high-speed feeder lines 10 and 12 are shown as being at the optical OC1 level and the electrical STS1 level respectively. Both of these feeder lines operate at 51.84 Mb/s. An add/drop multiplexer 14, as described in the aforementioned copending patent application, connects either feeder line 10 or 12 to a DS1 level transmission line 16 operating at 1.544 Mb/s. In order to extract individual DS0 channels from the DS1 line, a separate network element 15 usually referred to as a DLC was required as an interface between the DS0 level and the DS1 level. In a commercial installation several DLCs would be used with each add/drop multiplexer. Thus, two separate network elements 14 and 15 were required in order for individual lines at the DS0 level to access an optical OC1 level transmission line, and in most cases several DLCs were used with one multiplexer.
The need for a DS1 level transmission line between the ADM 14 and the DLC 15 severely limited the control communications that could take place between the two network elements. The use of two separate network elements inherently reduced reliability, since all control information had to be conveyed over a single DS1 line. Control information in the form of signaling could be conveyed only to a limited extent using the robbed bit signaling technique and clear channel communication was not available. A means did not exist for the microprocessors in each element to communicate with each other unless a separate DS0 channel was dedicated to this purpose, removing this particular DS0 channel from the pool of DS0 channels available to carry subscriber traffic.
The DLCs are generally adapted to accept a plurality of line units servicing one or more DS0 channels each. The line units are connected to a TDM bus with the line signals being multiplexed onto the bus at specified time slots, each line unit having predetermined time slots for inserting and extracting data to and from the bus for each of its channels. The line unit would be strobed to be activated during the time slots to which it is assigned. A plurality of back plane connections were required to provide a strobe signal to each line card. Thus, the system was limited to a pre-set number of line cards that could be handled by the system, and the assignment of the time slots to the line cards was fixed.
Control of the system was limited, since the system controller did not have access to the data contained within each time slot, including signaling information pertaining to the time slots.
With the advent of many different types of telecommunications service, various signaling schemes have been devised which are not compatible with each other. Signaling translation between equipment using one signaling scheme and equipment using another signaling scheme was required and the use of special signaling translation circuits for each different type of service had to be developed. Thus, the free interchange of line cards was restricted, due to the need to first provide for the required signaling translation.
Modern telecommunications systems must provide for high-speed wide bandwidth data communications facilities which cannot be serviced by individual DS0 level lines. Thus, higher speed DS1 lines had to be provided to handle individual customer requirements when they exceeded the capability of a DS0 line.
Thus, it was apparent that the flexibility demanded by modern telephone communications systems was not being efficiently handled by the prior art interfaces between high-speed feeder lines and DS0 level subscriber lines. There was clearly a need for a more efficient way of providing access to high-speed transmission lines by individual DS0 subscriber lines.